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Abstract

The Upper Rhine Graben (URG) in SW-Germany, a classical hydrocarbon province, is part of the European Cenozoic Rift System. Rift graben development has led to a complex basin fill of fluviatile-limnic terrestrial to brackish-marine deposits, providing several hydrocarbon reservoir and source rock units. Several studies have been carried out on palaeoenvironment conditions, source rock development and the thermal history in the URG, yet the hydrocarbon potential of source rocks within the northern URG is not fully understood and investigations concerning the thermal evolution are partly contradictory. The new methodological approach that is applied in this theses aims to investigate the impact of the rift-tectonic activity on the sedimentation history, the development of source rocks and the thermal evolution of the basin. To address this scientific interrogation, a wide set of methodologies was applied on rock material from former and recent hydrocarbon exploration wells. Regionally small-scale differences in palaeoenvironmental conditions are observed. Besides the proximity to the graben shoulders, also the structural position largely determines the organic material composition and oxygen availability. Thereby, footwall structures tend to obtain more oxic conditions than hanging wall structures, where more anoxic conditions are often present. These differences are tectonically induced due to variable subsidence rates on the different fault blocks and the coherently available accommodation space. Source rocks are restricted to the politic units of marine transgressive intervals. Two types of soruce rocks are identified: (i) Transgressive marine intervals during times of high rift tectonic activity show high subsidence and therefore high terrestrial input from the uplifted graben shoulders. This led to terrestrial dominated kerogen and mainly gas-prone source rocks, which are unexpected for a transgressive interval. Even in the maximum transgressive interval (Rupel Clay Group), mainly terrestrial dominated (gas-prone) kerogen was observed due to the high rift tectonic activity, instead of mainly marine derived oil-prone kerogen as commonly expected. (ii) In contrast, marine transgressions in times of low rift tectonic activity (e.g. Hydrobia Group) and low subsidence show low terrigenous sediment input, leading to the deposition of mainly marine-brackish originated, oil-prone kerogen, as typically expected for marine intervals. Thus kerogen composition is primarily linked and majorly controlled by the intensity of rift-related tectonic activity and only in second order by sea level variations. Minor differences in the kerogen composition within these intervals are linked to the different structural positions within the rift system. For a better understanding of the petroleum system in the northern URG the palaeothermal history was studied by integrated maturation analysis of several wells across the study area. Most wells show vertically almost uniform maturation trends based on the applied optical and geochemical methods. These trends are untypical for burial controlled maturation and clearly indicate significant secondary thermal overprint. By using one-dimensional numerical simulations, burial-controlled subsidence trends with high (90–100 mW/m2)/low (72–75 mW/m2) heat flows corresponding to phases of high/low rifting activity, could not be reconstructed using the obtained maturation data. Due to the absence of volcanic activity in the study area, these thermal anomalies must be related to long-lasting, very hot hydrothermal fluid systems, well known from the URG, which were mainly concentrated along reactivated fault zones. Therefore, at least in the vicinity of fault systems, maturation in the northern URG is mainly influenced by tectonically controlled distribution of hydrothermal systems and much less by basin subsidence. Taking into account both kerogen composition and thermal history of the graben, the best hydrocarbon potential can be expected from the highly oil-prone Hydrobia Group and the oil- to gas-prone Rupel Clay Group along the eastern graben border and within a small pull-apart basin in the northwest of the study area. From the results of the study it can be concluded, that the development of the depositional setting, kerogen composition, thermal maturation and hydrocarbon potential is directly linked and mainly controlled by the geotectonic changes within the rift system.